1. Field of the Invention
The present invention relates to novel fungi having improved capacity for secretion of recombinant polypeptides, and a method for improving such secretion.
2. Description of the Related Art
Filamentous fungi have become established as widely used host cell systems for the production of recombinant polypeptides. In many cases, however, fungi which have the desirable traits of ease of transformability and heterologous polypeptide expression do not necessarily have the most desirable characteristics for successful fermentation. For example, growth morphology during fermentation may not be optimal, since many cultures become quite viscous as biomass increases. Increased viscosity limits the ability to mix and aerate the fermentation culture, leading to oxygen and nutrient starvation of the mycelia, which in turn become inviable and unproductive limiting the yield of the polypeptide of interest. On the other hand, filamentous fungal strains showing good fermentation morphology are not necessarily the best production strains in terms of quantity of enzyme produced. Therefore, for commercial purposes, there is a need for filamentous fungal hosts which combine the capacity for expression of commercial quantities of recombinant polypeptide with satisfactory growth characteristics, such as rapid growth and low viscosity, thereby enhancing productivity during fermentation.
Screening of large numbers of mutants for improved fermentation morphology is quite difficult, and morphological mutant strains have often been isolated based on unusual colony morphology on solid medium. Traditionally, morphological mutants have been isolated in transformed strains that contain multiple copies of a heterologous gene. The mutants are then analyzed for fermentation growth characteristics and heterologous gene expression. Although this method may be useful in identifying improved expression strains, the relationship between any particular fungal growth morphology and a strain's ability to produce a large quantity of secreted polypeptide has yet to be established. Morphological mutants are also occasionally recovered in polypeptide expression improvement screens, following mutagenesis of a transformed strain, but again, the study of these strains has not led to any significant insight into the control of morphology. In addition, morphologically "improved" strains of parental strains containing heterologous gene expression cassettes are not suitable as general expression hosts since they cannot be used for the exclusive expression of other heterologous polypeptides.
It is an object of the present invention to provide methods for producing and identifying useful morphological mutants for heterologous polypeptide production.